Recent estimations indicate that, on the average, each airplane in the U.S. commercial aircraft fleet receives a lightning discharge about once each year. The airplane generally experiences the lightning discharge when flying through a heavily charged portion of a cloud. In such cases, the discharge generally originates at the airplane and extends outwardly from the aircraft. While the discharge is occurring, it generally moves from the nose of the airplane and onto a plurality of skin panel portions of the airplane as it moves through the charged region. The discharge may also attach to wing tips and/or edges of wing control surfaces (e.g., ailerons) during the discharge. The discharge then generally leaves the aircraft structure through the empennage. Since commercial airplanes contain relatively large amounts of potentially-explosive fuel, and also generally include sensitive electronic equipment such as navigational computers and communications equipment that may be easily damaged by a lightning discharge, commercial airplanes are required to comply with a comprehensive set of certification procedures in order to verify that the airplane is sufficiently protected from the damaging effects of a lightning discharge.
Presently, the outer skin panels in commercial airplanes are formed from an aluminum alloy, which is an excellent electrical conductor. Accordingly, by providing suitable electrical interconnections between the outer skin panels, and between other exposed portions of the aircraft structure, the current associated with the discharge is safely communicated along the skin panels and/or other structural portions as the airplane moves through the charged region. Increasingly, however, structural portions (including skin panel portions) of commercial airplanes are formed from fiber-reinforced composite materials, which are relatively poor electrical conductors. Consequently, additional conductive materials are generally incorporated into the fiber-reinforced composite materials so that adequate lightning discharge protection is achieved.
In particular, skin panels fabricated from fiber-reinforced composite materials generally include a conductive material, such as a conductive mesh, or other similar conductive structures that may be applied to surfaces of the skin panels so that the lightning current is safely communicated through the skin panels. Alternately, the conductive mesh or conductive structure may be incorporated into one or more of the layers of the skin panels.
Although the foregoing fiber-reinforced composite panels are generally sufficiently conductive, electrical conductivity must also be maintained between each of the panels in order to provide adequate lightning protection to the airplane. Accordingly, in one known method, respective edges of abutting skin panels are sanded or etched to expose edges of the conductive mesh provided in the panels. A suitably-sized and pre-assembled surface member having a conductive mesh may then is positioned on the sanded or etched portions of the abutting panels and bonded to the panels to provide electrical continuity between the abutting panels. Similarly, fiber-reinforced composite skin panels may also require a localized repair to remove a portion of the panel that has been physically damaged. The localized repair includes removing the damaged portion of the panel, and fabricating a patch that covers the removed portion. Since electrical continuity between the patch and the surrounding skin panel must be established, edge portions of the panel and the patch must be suitably prepared, which generally includes sanding or etching the edge portions, and bonding the patch to the skin panel, as described above.
To assure that suitable electrical continuity exists between the pre-assembled surface member or repair patch and the surrounding skin panel material, direct electrical conductivity measurements using a surface conductivity probe have been used. In one known method, conductivity measurements are made at various locations around the pre-assembled surface member or repair patch and suitably processed to determine if the required conductivity is attained. Although this method provides an estimation of the surface conductivity of an affected area, producing an estimation of the conductivity requires a number of independent measurements to be made by a skilled operator. Accordingly, surface conductivity measurement methods are tedious and are prone to error if not performed correctly.